1. Field of the Invention
The invention relates to a magnetic-inductive flow meter, with at least one measurement tube for through-flow of an electrically conductive medium, with at least one magnetic field generating apparatus for generating a magnetic field which runs at least also perpendicular to the longitudinal axis of the measurement tube, and with at least two measurement electrodes, the measurement tube having a metallic base body and the base body being provided with a thermoplastic cover layer at least on the inside of the measurement tube, and the virtual connecting line of the two measurement electrodes running at least essentially perpendicular to the direction of the magnetic field which is permeating the measurement tube perpendicular to the longitudinal axis of the measurement tube. The invention also relates to a method for producing such a magnetic-inductive flow meter.
2. Description of Related Art
Magnetic-inductive flow meters have been widely known in the prior art for decades. Reference is made by way of example to the literature citation Technical Flow Rate Measurement by Dr. Eng. K. W. Bonfig, 3rd edition, Vulcan-Verlag Essen, 2002, pp. 123 to 167 and moreover to the literature citation Principles of Magnetic-Inductive Flow Rate Measurement by Cert. Eng. Friedrich Hoffmann, 3rd ed., publication of the company KROHNE Messtechnik GmbH & Co. KG, 2003.
The basic principle of a magnetic-inductive flow meter for measuring the flow rate of a flowing medium goes back to Michael Faraday who suggested the use of the principle of electromagnetic induction for measuring the flow velocity of an electrically conductive medium as early as 1832.
According to the Faraday Induction Law, in a flowing, electrically conductive medium which is permeated by a magnetic field, an electric field intensity arises perpendicular to the flow direction of the medium and perpendicular to the magnetic field. The Faraday Induction Law is used in magnetic-inductive flow meters in that, by means of a magnetic field generating apparatus which has at least one magnetic field coil, conventionally two magnetic field coils, a magnetic field which changes over time during a measurement process is generated and the magnetic field at least partially permeates the electrically conductive medium which is flowing through a measurement tube. The generated magnetic field has at least one component perpendicular to the longitudinal axis of the measurement tube and perpendicular to the flow direction of the medium.
It was stated at the beginning that the measurement tube has a metallic base body and the base body is provided with a thermoplastic cover layer on at least the inside of the measurement tube. Instead of such a measurement tube, there can also be a measurement tube which, instead of a metallic base body, has a nonmetallic base body, for example, a ceramic base body. Here, magnetic-inductive flow meters will also be encompassed in which the measurement tube is formed entirely of a thermoplastic material. But, it is always assumed below that the measurement tube has a metallic base body and the base body tube is provided with a thermoplastic cover layer on at least the inside of the measurement. The formulation “at least on the inside of the measurement tube” of course also comprises an embodiment in which the base body is provided on all sides with a thermoplastic cover layer.
With regard to the statement at the beginning that the magnetic-inductive flow meter under discussion includes at least one magnetic field generating apparatus “for producing a magnetic field which runs at least also perpendicular to the longitudinal axis of the measurement tube”, it is pointed out again here that the magnetic field does run preferably perpendicular to the longitudinal axis of the measurement tube and perpendicular to the flow direction of the medium, but it is sufficient that one component of the magnetic field runs perpendicular to the longitudinal axis of the measurement tube and perpendicular to the flow direction of the medium.
It was also stated at the beginning that the magnetic-inductive flow meter under discussion includes at least two measurement electrodes, the virtual connecting line of the two measurement electrodes running at least essentially perpendicular to the direction of the magnetic field which is permeating the measurement tube. Preferably, the virtual connecting line of the two measurement electrodes in fact runs more or less perpendicular to the direction of the magnetic field which permeates the magnetic field [sic].
The electrical field intensity which is produced by induction in the flowing, electrically conductive medium can be measured by measurement electrodes which are directly, therefore electrically in contact with the medium as electrical voltage or also can be capacitively detected by electrodes which are not directly, therefore not electrically in contact with the medium. Here it is a matter of magnetic-inductive flow meters in which the electrical field intensity produced by induction in the flowing, electrically conductive medium is measured by measurement electrodes which are directly, therefore electrically in contact with the medium as electrical voltage.
The measurement error in the magnetic-inductive flow meters known from the prior art is relatively small today; a measurement error less than 0.2% can be accomplished.
For the known magnetic-inductive flow meters, reference is made by way of example to the German patent disclosure document 197 08 857, 10 2004 063 617 and corresponding U.S. Pat. No. 7,261,001, German patent disclosure document 10 2008 057 755 and corresponding U.S. Pat. No. 8,286,503, German patent disclosure document 10 2008 057 756 and corresponding U.S. Pat. No. 8,286,502 and commonly owned, unpublished pending U.S. patent application Ser. No. 13/687,313. The disclosure content of the aforementioned documents which were published beforehand are hereby expressly in corporate by reference in this patent application as is the substance of the co-pending U.S. patent application Ser. No. 13/687,313.